1. Field of the Invention
The present invention relates to an electrodeionization apparatus and method and, more particularly, to an electrodeionization apparatus and method including electroactive media combinations that provide more uniform electric current distribution, degree of resin regeneration, and deionization performance.
2. Description of the Related Art
Electrodeionization (EDI) is a process that removes ionizable species from liquids using electrically active media and an electrical potential to influence ion transport. The electrically active media may function to alternately collect and discharge ionizable species, or to facilitate the transport of ions continuously by ionic or electronic substitution mechanisms. EDI devices may comprise media of permanent or temporary charge, and may be operated batchwise, intermittently, or continuously. EDI devices may be operated to cause electrochemical reactions specifically designed to achieve or enhance performance, and may comprise electrically active membranes such as semipermeable ion exchange or bipolar membranes.
In continuous electrodeionization (CEDI), which includes processes such as continuous deionization, filled cell electrodialysis, or clectrodiaresis (EDR), the ionic transport properties of the electrically active media are the primary sizing parameter. These processes are described, for example, by Kollsman in U.S. Pat. No. 2,815,320; Pearson in U.S. Pat. No. 2,794,777; Kressman in U.S. Pat. No. 2,923,674; Parsi U.S. Pat. Nos. 3,149,061 and 3,291,713; Korngold et al. in U.S. Pat. No. 3,686,089; Davis in U.S. Pat. No. 4,032,452; U.S. Pat. No. 3,869,376; O'Hare in U.S. Pat. No. 4,465,573; Kunz in U.S. Pat. Nos. 4,636,296 and 4,687,561; and Giuffrida et al. in U.S. Pat. No. 4,632,745. A typical CEDI device comprises alternating electroactive semipermeable, anion and cation ion-exchange membranes. The spaces between the membranes are configured to create liquid flow compartments with inlets and outlets. A transverse DC electrical field is imposed by an external power source using electrodes at the bounds of the membranes and compartments. Often, electrolyte compartments are provided so that reaction products from the electrodes can be separated from the other flow compartments. Upon imposition of the electric field, ions in the liquid are attracted to their respective counterelectrodes. The compartments bounded by the electroactive anion membrane facing the anode and the electroactive cation membrane facing the cathode become ionically depleted, and the compartments bounded by the electroactive anion membrane facing the cathode and the electroactive cation membrane facing the anode become ionically concentrated. The volume within the ion-depleting compartments, and preferentially within the ion-concentrating compartments, is also comprised of electrically active media. In continuous deionization devices, the media may comprise intimately mixed anion and cation exchange resins. The ion-exchange media enhances the transport of ions within the compartments and can also participate as a substrate for controlled electrochemical reactions. The configuration is similar in EDR devices, except that the media comprise separate, and sometimes alternating, layers of ion-exchange resin. In these devices, each layer is substantially comprised of resins of the same polarity (either anion or cation resin) and the liquid to be deionized flows sequentially through the layers.
Performance of CEDI may be limited by difficulty in obtaining the desired electrical current distribution with the device. Electroactive media of permanent charge may change their electrical resistance properties in undesired ways depending on their ionic form. For example in the ion substitution of sodium with hydrogen ion in EDR, most cation exchange resins will preferentially transport hydrogen ion over the desired transport of sodium ion. This results in electrical inefficiencies and, under certain circumstances, may cause pH shifts that are detrimental to valuable products within the liquid. In another example, a given electroactive media may be desirable for transport properties, such as the Type II anion membrane and resins for continuous deionization and EDR, but may have the undesirable properties of catalyzing the ionization reaction of water to hydrogen and hydroxide ions.
NL 776,469 discloses an electrolytic deionization apparatus that includes separate strata of anion exchanger and cation exchanger.
WO97/46492 discloses an electrodeionization apparatus for improving the rate of silica removal in which nontreated water first passes through an anion-exchange layer followed by other ion-exchange materials.
Therefore, a need remains for an improved electrodeionization apparatus having improved electric current distribution and deionization performance. Lastly, there is a need to provide improved methods of operation of an electrodeionization apparatus.